PS question

Having an odd problem with photoshop.

I create a new 256x256, 72dpi, RBG image
I fill the foreground with black
I bicubic image-size it to 512x512
then i bicubic image-size it to 256x256

Photoshop is essentially making a 1 pixel border semi-transparent (which comes out in a tga as a lighter edging)

It isn't easily noticeable just on an image itself, but when you take it into SL and try to tile/repeat it, the "border" becomes obvious

anyone know the root of the problem (hopefully i've explained it clearly)

I've duplicated your result and have an explanation. You're seeing something akin to a rounding error. The upsample to 512 works fine, the downsample is a problem because of the way bicubic reduction works. Bicubic resampling takes the average of the 16 pixels surrounding the destination pixel; for an edge pixel, that means that half of the 16 pixels are null and so you wind up with a half black quasi-anti-aliased edge.

I'm not sure why you are doing this, but here are two workarounds. First, you could use a linear resample which works fine for a block of black, but I'm guessing you arent working on that. Alternately, you can bicubic resample down to 258x258 and trim off the top, bottom and side pixels using Image->Canvas size which will warn of clipping but will get rid of the fuzz.

Good luck.

thanks malachi... well the example above was just to simply and clearly show the effect i'm getting.

what i'm actually doing is trying to take a seamless 512x512 texture (say, wood planks) and shrink it to 256x256 for better in-world performance

the problem with doing 258x258 and then clipping, is that it causes problems with the seamless nature of the texture.

Forseti - obviously you would start with 256x256 if you could, right? so, I'm guessing these are some thrid-party supplied textures. as such, they are hopefully just simple, single-layered images.
so: it's a bit of work, but you should be able to set up a photoshop action to:

1. change the canvas size to 3 time wider and 3 times higher
   
2. create an additional 8 copies of the layer, position them so that you hva e 3x3 seamless grid
   
3. resize
   
4. set the canvas size to 1/3 and 1/3
   

there are other variants to this, but basically you want to give the edge pixels neighbours (from the other side of the image) for them to seamlessly interpolate with.

You can get a nice, clean edge for your resampled images if you do the following *before* you scale them (up or down):

1. Ctrl+A (select all), Image->Crop
   
2. Layer->Flatten Image
   

Step 1 is not always necessary.

With those, you're basically forcing Photoshop not to take offscreen pixels into account when it is performing any calculations.

thank you all.

Ari -- you are right, if I flatten the image so photoshop calls it background rather than layer 0, then it resizes without the fringe. Excellent!